A Fresnel lens is a regular array of refracting facets arranged according to whether the desired result is to form a lens of spherical or cylindrical symmetry. The refracting facets correspond to segments of the spherical or cylindrical lens of equivalent focal length. These segments are curved in an exact replica of the spherical or cylindrical lens segments, but in practice they are tangents to the curved surfaces of the individual segments, an approximation which is acceptable if the number of refracting facets per unit radius of the corresponding spherical or cylindrical lens is large enough. Typically a Fresnel lens will have from about 15 to 60 such elements per centimeter of radius of the lens.
For a solar energy concentrator the Fresnel lens must be cheap and have a long maintenance-free life. The cost of plastic Fresnel lenses presently available is too high to allow solar concentration techniques to be competitive with other power sources except in specialized applications. Plastic Fresnel lenses have the additional problems that they are susceptible to ultraviolet radiation degradation, are dimensionally unstable and have a high coefficient of thermal expansion, which in turn produces mounting and distortion problems. They are also soft and therefore susceptible to abrasion from wind blown particles and, during cleaning procedures, from particles adhering to the surface. These problems both increase the initial cost of such concentrators and shorten their useful life.
Jebens, in a copending application referred to above and incorporated herein by reference, has described a Fresnel lens which comprises a thin plastic layer, in which are formed the refractive facets, bonded to a transparent substrate. The substrate is low cost, dimensionally stable, has a low coefficient of thermal expansion, and is hard and therefore not susceptible to abrasion. The plastic layer is thin so that any increase in its absorption due to ultraviolet degradation does not result in significant light loss. Since the lens can be arranged with the plastic layer bonded to the side of the substrate opposed to the weather, the plastic layer is protected from abrasion by wind blown or adhering particles.
This novel Fresnel lens is a significant improvement over the prior art and will enhance the utility of the solar energy concentrator. Thus, a method of fabrication which would produce a lens with a plastic layer of minimum thickness and which would be economical and lend itself to large scale production would be most desirable.